Strip mining is a method of removing the waste rock from above the ore body in strips to expose the ore to be mined and depositing the waste rock in the void of the previously mined strip. The strip mining method usually starts with a box cut, which removes the waste rock from above the ore body to another location. The ore is mined and the miners move to another strip. With the new strip the waste rock is deposited in the void created from the previously mined strip. The method is repeated as the mining operation continues to advance.
The amount of material from a particular strip that can be moved by dragline or bulk doze methods is limited by the spoil room available in the previous strip void in which to deposit the material. Truck shovel is a more expensive method, however it is not limited by spoil room in the previous void as the truck can haul the material out of the excavation to an alternate dump location.
The conventional strip mining method uses a series of steps of uncovering the seam of coal using cast blasting, bulk dozer push and truck/excavator stripping. This is shown in the FIGS. 1-4.
With reference to FIG. 1, a seam of coal 1 is overlain by a waste burden 2 that is several times the thickness of the coal 1. The direction of stripping advance is to the right in the section.
In FIG. 2, a strip of waste is blasted and some material is thrown by the blast into a position 3 where it is considered to be in final spoil because it does not have to be moved further in order to uncover and mine the coal. Position 3 is in the void 6 from the previous strip, which is adjacent to the spoil 7 from the previous strip.
The dozers 14 then push waste material across the low wall batter line 4 into final spoil (see FIG. 3). The productivity of the dozer 14 declines as the uphill angle and distance of the push increases. As the productivity of the dozer 14 declines, the unit cost of moving waste material with the dozer increases proportionately. The lowest overall cost outcome is obtained when the dozer 14 ceases pushing material when the cost of pushing material is equivalent to the cost of using an excavator 16 and truck 5 to remove the material. Area 7 indicates the material that is moved by the dozer 14. The remainder of the waste material is then moved by an excavator 16 and trucks 5.
As shown in FIG. 4, an excavator 16 digs out the remaining waste, placing it into trucks 5 which haul the material to a waste dump. The excavator and truck fleet is supported by a dozer to clean up the dig face, a dozer to maintain the waste dump, a grader to maintain the road surface and a water truck to suppress dust on the haul road. Due to the large number of machines involved and the relatively long material movement distance, stripping with an excavator and trucks is typically more expensive per unit of material moved than stripping by a dozer.
The commercial viability of mining a particular site depends on the cost of removing the waste material to provide access to the ore layer. The need to employ more expensive methods and equipment of removing waste material lowers the commercial viability of the mining operation.